Platinum Monolayer Research Articles

Electrodeposition of Metals in Catalyst Synthesis: The Case of Platinum Monolayer Electrocatalysts

The article describes several metal electrodeposition techniques in synthesis of a new class of electrocatalysts comprising a platinum monolayer on metal or alloy nanoparticles. These electrocatalysts can dramatically reduce the Pt content, while affording enhanced catalytic activity. The galvanic displacement of a Cu underpotential deposition monolayer by Pt provides a uniquely convenient way of uniformly depositing a metal monolayer on carbon-supported metal nanoparticles in a surface-limited reaction controllable to a small fraction of a monolayer. The environmental benefits resulting from the use of fuel cells in transportation would be an especially important result of the application of Pt monolayer electrocatalysts.

Platinum Monolayer Electrocatalysts for the Oxygen Reduction Reaction: Improvements Induced by Surface and Subsurface Modifications of Cores

This paper demonstrates that the ORR activity of PtML electrocatalysts can be further improved by the modification of surface and subsurface of the core materials. The removal of surface low‐coordination sites, generation (via addition or segregation) of an interlayer between PtML and the core, or the introduction of a second metal component to the subsurface layer of the core can further improve the ORR activity and/or stability of PtML electrocatalysts. These modifications generate the alternation of the interactions between the substrate and the PtML, involving the changes on both electronic (ligand) and geometric (strain) properties of the substrates. The improvements resulted from the application of these approaches provide a new perspective to designing of the new generation PtML electrocatalysts.

Enhanced Oxygen Reduction Activity of Platinum Monolayer on Gold Nanoparticles.

The increase in oxygen binding energy was previously proposed to account for the lower oxygen reduction activity of a Pt monolayer supported on Au(111) single crystal than that on Pd(111) and pure Pt(111) surfaces. This single-crystal based understanding, however, cannot explain the new finding of a 1.6-fold increase of oxygen reduction activity on Pt monolayer-modified 3-nm Au nanoparticles (Pt/Au/C) in comparison with that on Pt/Pd/C with a similar particle size. The Pt/Au/C catalyst also has an activity higher than that of a state-of-the-art 2.8-nm Pt/C catalyst. Our new experimental results and density functional theory calculations demonstrate that a significant compressive strain in the surface of the core nanoparticles plays a role in the observed activity enhancement.

Platinum Monolayer Electrocatalysts for O2 Reduction: Pt Monolayer on Carbon-Supported PdIr Nanoparticles

The kinetics of oxygen reduction was investigated in acid solutions on Pt monolayers deposited on modified carbon-supported PdIr nanoparticles using the rotating disk-electrode technique. Iridium is introduced into the Pd substrate in order to fine-tune the Pt–Pd interactions and to improve Pd stability under operating conditions of the fuel cell. The kinetics of the oxygen reduction reaction shows enhancement with the Pt monolayer on the PdIr nanoparticle surfaces in comparison with the reaction on Pt/C and Pt monolayer on Pd/C nanoparticles. The electrochemical measurements suggest that reduced oxidation of Pt monolayer on PdIr/C compared to Pt/C and Pt monolayer on Pd/C is the cause of enhanced activity. Besides a ligand effect induced to the Pt surface by the presence of PdIr in the second sublayer of the nanoparticle, as suggested by our density functional theory analysis, Ir also leads to a Pd skin contraction, so the Pt monolayer on PdIr/C is compressed more than on Pd/C. Both effects lead to further weakening of the Pt–OH interaction, thus increasing the ORR activity. The Pt-specific activity for PtMLPdIr/C is three times and 25% higher than that of Pt/C and PtMLPd/C respectively; the Pt-mass activity of PtMLPdIr/C is more than 20 times and 25% higher than that of Pt/C and PtMLPd/C, respectively.

Core‐Protected Platinum Monolayer Shell High‐Stability Electrocatalysts for Fuel‐Cell Cathodes

More than skin deep: Platinum monolayers can act as shells for palladium nanoparticles to lead to electrocatalysts with high activities and an ultralow platinum content, but high platinum utilization. The stability derives from the core protecting the shell from dissolution. In fuel-cell tests, no loss of platinum was observed in 200?000 potential cycles, whereas loss of palladium was significant.

Core‐Protected Platinum Monolayer Shell High‐Stability Electrocatalysts for Fuel‐Cell Cathodes

Core‐Protected Platinum Monolayer Shell High‐Stability Electrocatalysts for Fuel‐Cell Cathodes

Bimetallic Ni-Ir Core Platinum Monolayer Shell Electrocatalysts for the O2 Reduction Reaction

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Platinum-monolayer electrocatalysts: Palladium interlayer on IrCo alloy core improves activity in oxygen-reduction reaction

We describe the synthesis and electrocatalytic properties of a new low-Pt electrocatalyst consisting of an IrCo core, a Pd interlayer, and a surface Pt monolayer, emphasizing the interlayer’s role in improving electrocatalytic activity for the oxygen-reduction reaction on Pt in HClO 4 solution. We prepared the IrCo alloys by decomposing, at 800 °C, hexacyanometalate, KCoIr(CN) 6, adsorbed on the carbon surfaces. The synthesis of Ir 3Co/C involved heating a mix of metal salts and carbon in hydrogen at 500 °C. Thereafter, we placed a palladium and/or platinum monolayer on them via the galvanic displacement of an underpotentially deposited copper monolayer. The electrocatalysts were characterized using structural- and electrochemical-techniques. For Pt ML/Pd ML/IrCo/C, we observed a Pt mass activity of 1.18 A/mg (Pt) and the platinum-group-metals’ mass of 0.16 A/mg (Pt, Pd, Ir). In comparison, without a Pd interlayer, i.e., Pt ML/IrCo/C, the activities of 0.15 A/mg (Pt) and 0.036 A/mg (Pt, Pd, Ir) were considerably lower. We consider that the palladium interlayer plays an essential role in achieving high catalytic activity by adjusting the electronic interaction of the platinum monolayer with the IrCo core, so that it accelerates the kinetics of adsorption and desorption of the intermediates of oxygen reduction. A similar trend was observed for Pt ML/Pd ML and Pt ML deposited on Ir 3Co/C alloy core. We used density functional theory to interpret the observed phenomena.

Characterization of Platinum Monolayer Electrocatalysts in Long-Term Fuel Cell Cathode Tests

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Platinum Monolayer Electrocatalysts for O2 Reduction: Pt Monolayer on Carbon-Supported PdIr Nanoparticles

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Surface segregation and stability of core–shell alloy catalysts for oxygen reduction in acid medium

Density functional theory is used for the evaluation of surface segregation, trends for dissolution of Pt surface atoms in acid medium, and oxygen reduction reaction activity of core-shell materials, containing a monolayer of platinum over a monometallic or bimetallic core. Two groups of cores are investigated: Pt/X with X = Ir, Au; Pd, Rh, Ag; Co, Ni, Cu; and Pt/Pd(3)X, with X = Co, Fe, Cr, V, Ti, Ir, Re. It is found that all the 4d and 5d pure cores may serve as stable cores, and their beneficial effect on the Pt monolayer may be further tuned by alloying the core to another element, here chosen from 3d or 5d groups. The Pd(3)X cores enhance the stability of the surface Pt atoms both in vacuum and under adsorbed oxygen; however the high oxygen philicity of some of the X elements induces their surface segregation that may cause surface poisoning with oxygenated species and their dissolution in acid medium.

Recent advances in platinum monolayer electrocatalysts for oxygen reduction reaction: Scale-up synthesis, structure and activity of Pt shells on Pd cores

We have established a scale-up synthesis method to produce gram-quantities of Pt monolayer electrocatalysts. The core–shell structure of the Pt/Pd/C electrocatalyst has been verified using the HAADF-STEM Z-contrast images, STEM/EELS, and STEM/EDS line profile analysis. The atomic structure of this electrocatalyst and formation of a Pt monolayer on Pd nanoparticle surfaces were examined using in situ EXAFS. The Pt mass activity of the Pt/Pd/C electrocatalyst for ORR is considerably higher than that of commercial Pt/C electrocatalysts. The results with Pt monolayer electrocatalysts may significantly impact science of electrocatalysis and fuel-cell technology, as they have demonstrated an exceptionally effective way of using Pt that can resolve problems of other approaches, including electrocatalysts’ inadequate activity and high Pt content.

Some properties of electrochemical nanostructures

The physical and electronic properties of several platinum nanostructures have been investigated by density functional calculations. Particular attention has been paid to the structure of the d-band. Our results predict, that nanowires and small platinum clusters supported on Au(111) should be excellent catalysts for the hydrogen evolution reaction; a monolayer of platinum on Au(111) should also be better than pure platinum.

Fuel Cell Performance and Long Term Stability of Platinum Monolayer Electrocatalysts

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Platinum Monolayer Electrocatalysts: Tuning the Pt Monolayer-Supporting Core Interaction by a Pd Interlayer

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Promoted Catalytic Activity of a Platinum Monolayer Cluster on Graphite

Platinum atoms deposited on a graphite surface show distinct different catalytic activity depending on their morphology on the surface. Flat monolayer clusters of platinum on graphite show higher catalytic activity than taller Pt clusters for the H2−D2 exchange reaction. It is suggested based on scanning tunneling microscopy observations and X-ray photoelectron spectroscopy measurements that the origin of the superiority is due to the interface interaction between Pt and graphite, leading to a shift of the d-band center of platinum away from the Fermi level.

Platinum Monolayer Electrocatalysts: Synthesis, Structure and Activity of Ideal Pt Shells on Pd Cores

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Platinum Monolayer Electrocatalysts: Tuning the Pt Monolayer-Supporting Core Interaction by a Pd Interlayer

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Improved oxygen reduction reactivity of platinum monolayers on transition metal surfaces

The catalytic activity of platinum monolayers supported on close-packed transition metal surfaces (Au(1 1 1), Pt(1 1 1), Pd(1 1 1) and Ir(1 1 1)) is investigated for the oxygen reduction reaction (ORR) by generating free energy diagrams and performing Sabatier analysis based on periodic, self-consistent density functional theory (DFT) calculations. Three different ORR mechanisms, involving direct or hydrogen-assisted activation of O 2, are considered. At the ORR equilibrium potential of 1.23 V, the reactivity of all surfaces is shown to be limited by the rate of OH removal from the surface. At a cell potential of 0.80 V, the ORR reactivity of different surfaces is dictated by the strength of oxygen adsorption, with OH removal via hydrogenation and O–O bond scission in either O 2, O 2H or H 2O 2 being the rate-limiting steps for surfaces with stronger and weaker oxygen binding, respectively. Among the surfaces studied, Pt monolayer on a Pd(1 1 1) substrate shows the highest reactivity and is more active than Pt(1 1 1). These results are in excellent agreement with our earlier experimental and theoretical work, which was based on a simpler model for the ORR.

Platinum Monolayer Fuel Cell Electrocatalysts

We describe a new class of electrocatalysts for the O2 reduction, and H2 and methanol oxidation reactions, consisting of a monolayer of Pt deposited on a metal or alloy carbon-supported nanoparticles. These electrocatalysts show up to a 20-fold increase in Pt mass activity compared with conventional all-Pt electrocatalysts. The origin of their increased activity was identified through a combination of experimental methods, employing electrochemical and surface science techniques, X-ray absorption spectroscopy, and density functional theory calculations. The long-term tests in fuel cells demonstrated excellent stability of the anode and good stability of the cathode electrocatalysts. We also describe the stabilization of Pt electrocatalysts against dissolution under potential cycling regimes effected by a submonolayer of Au clusters deposited on Pt surfaces. These new electrocatalysts promise to alleviate some of the major problems of existing fuel cell technology.